Polyester monofilaments having increased resistance to hydrolytic and thermal degradation have long been sought in the monofilament industry. Such monofilaments have been found to be suitable components for use in fabrics in the paper making industry.
Traditionally, these monofilaments have been prepared by melt extruding standard polyester resins such as polyethylene terephthalate (PET). This polyester is well-known in the art and has long been used in the production of monofilaments which are suitable for use in the manufacture of paper machine fabrics. PET has a known melting point of less than 260.degree. C. and can be readily adapted for monofilament use. However, while PET has relatively good dry heat (thermal) stability, it has only moderate hydrolytic stability as compared to polyester resins having higher melt temperatures.
Accordingly, attempts have been made to improve the hydrolytic stability of PET. For example, Barnewall U.S. Pat. No. 3,975,329 indicates that the hydrolytic as well as the thermal stability of PET can be improved by melt extruding this standard polyester resin in the presence of a significant amount of a carbodiimide. Specifically, the patent indicates that the amount of carbodiimide used should be equal to the concentration of carboxyl groups in the original resin plus the concentration of carboxyl groups generated when the original resin is extruded in the absence of carbodiimide.
Notably, high temperature polyester resins having a melting point above about 260.degree. C. have been found to be suitable for use in monofilaments for paper machine fabrics. Such high temperature polyester resins include, but are not limited to poly(cyclohexanedimethylene terephthalate) and poly(cyclohexanedimethylene terephthalate/ isophthalate). The use of poly(1,4-cyclohexanedimethylene terephthalate/isophthalate) and its suitability for use in paper machine fabrics is essentially described in Eagles U.S. Pat. No. 5,169,499. It is referred to as poly(1,4-cyclohexanecarbinyl terephthalate) in this patent. Specifically, the Eagles patent discloses the use of a resin comprising carboxyl groups which are hindered by a moiety selected from cyclic aliphatic and branched aliphatic glycols and is noted as having excellent hydrolytic stability. However, this high temperature polyester resin has lower dry heat (thermal) stability as compared to conventionally stabilized PET.
Notwithstanding these improvements, a need still exists, as a result of the deleterious conditions which paper machine fabrics are subject to during the paper making process, for improved polyester resin blends which may produce suitable monofilaments having improved resistance to both hydrolytic and thermal degradation.